The modulation of optical radiation for the transmission of information has become more attractive with advances in the processing of the optical radiation. The large bandwidth potentially available in the modulation of optical radiation has been one of the factors encouraging activity in this area.
One of the components of an information system using modulated optical radiation to convey information is an optical receiver. The present invention relates to an optical receiver that can recover an amplitude modulated (AM) or frequency modulated signal from the detected optical signal. The use of the receiver to recover amplitude or frequency modulated signals at video signal frequencies, as opposed, for example, to recovery of digitally encoded signals, imposes additional requirements on the linearity of the optical receiver.
Referring to FIG. 1a, FIG. 1b, and FIG. 1c, optical receivers, illustrative of the prior art, are shown. In each case, the output signal of photodetector diode 11 is applied to a video amplifier. In FIG. 1a, light applied to the photodetector 11 causes a photoelectric current to be generated. This current is substantially converted to a voltage by resistor 14 for application to an input terminal of a video amplifier 13. In this configuration, a relatively high thermal noise results when the resistance 14 is too low, while a large value of the resistance 14 drives the amplifier 13 into saturation. When intermediate values of resistance 14 are chosen, the linearity (e.g. level of distortion) of the receiver is compromised. This compromised linearity reduces the usefulness of this optical receiver for wideband applications, e.g. AM or FM video transmission, since undue crosstalk between video channels results.
In FIG. 1b, the photo detector 11 converts the time variation of light photon flux into a time variation of electric current. The amplifier 13 and feedback impedance 15 convert the time variation electronic current into a time variation of voltage. This configuration has the same disadvantages as the configuration illustrated in FIG. 1a due to the limitations imposed by thermal noise and saturation created by use of the resistor 15.
Referring now to FIG. 1c, the output current from the photodetector 11 is applied to the input terminal of the video amplifier 13 by means of transformer 16. This configuration suffers from non-linearity of the transformer 16, a characteristic which compromises the usefulness of this receiver in amplitude modulation applications. Again, the transformer tends to convert the time variation electronic current into a time variation of voltage.
Japanese Patent No. 63-136836 discloses a plurality of resonant frequency circuit stages which are coupled between a photodetector stage and a video amplifier. These plurality of stages enhance the bandwidth. As in FIGS. 1a and 1b, a resistor 4 is used to convert a time varying current to a time varying voltage, this resistor being subject to the thermal noise/saturation tradeoff noted above.
Japanese Patent No. 54-31791 discloses a filter circuit, including a capacitor coupling the voltage signal produced by the photodetector element to the input terminal of the video amplifier, and an inductance coupled from the input terminal of the video diode to the ground terminal. Again, resistor 3 is used to substantially limit an amount of current applied to the amplifier, this circuit thus again being subject to the thermal noise/saturation tradeoff.
A need has therefore been felt for a circuit, including a photodetector device, which can respond to modulated optical transmissions and convert the optical transmissions to electrical signals so as to produce lower distortion and higher sensitivity than heretofore has been available. Such a circuit is particularly desirable for the optical transmissions which have amplitude or frequency modulation imposed thereon and the electrical signals must accurately reflect the information imposed on the optical transmissions and yet be extremely sensitive.